1. Field of the Invention
This invention relates to an indirectly-heated cathode heater structure comprising a heat generation section for heating a cathode to emit thermions at an electron gun of a cathode ray tube.
2. Description of the Prior Art
Generally, a cathode body having a cathode heater (hereinafter referred to as a "heater" for brevity) mounted therein, and used in an electron gun of a cathode ray tube is of the type as shown in FIG. 1 of the accompanying drawings, which broadly comprises a cathode cap 2 having electron emission material 1 made of carbonate, and a sleeve 4 being a hollow body made of Ni--Cr alloy surrounding the heater 3. When a rated voltage is applied to the heater 3 of the electron gun, the temperature of a heat generation section of the heater 3 is raised to a given temperature in the range of about 780.degree.-820.degree. C. in 4 to 6 seconds. This heat is transferred to the sleeve 4 and the cathode cap 2 of the cathode body. Therefore, in order for the heater to have the given calorific value necessary for emitting the thermions of a predetermined density, the heater needs to have a calorific wire 5 and arrangement structure suitable for efficient heat generation.
The prior art heater used in the electron gun comprises, as shown in FIG. 1, the heat generation section 3a positioned inwardly of the upper portion of the sleeve 4, a power input section 3b positioned inwardly of the lower portion of the sleeve, and a fixing section 3c. In such a heater, to enhance the thermal efficiency of the heat generation section 3a to have the given calorific value, the heat generation section has a double wound structure and is disposed within the upper portion of the sleeve 4 over a length approximately 45% of the overall length L of the sleeve.
However, since the length of the heat generation section of the heater according the the prior art is unnecessarily longer than a desired length, considerable heat is lost at the lower portion of the sleeve. As a result, in an initial state just after application of voltage to the heater, the heater has an initial current which overshoots the reference current. This overshoot rate ranges from a minimum of 133% to a maximum of 157% of the reference current. This rate considerably deviates from the reference cathode current variation in the range of 100.about.120%, which corresponds to the overshoot rate allowable in the actual design. The term "overshoot" as referred to herein means an initial cathode current variation when the heater of the cathode ray tube has been initially energized, and "overshoot rate" means the percentage of the initial cathode current value relative to the reference cathode current value.
The graph of FIG. 2 shows the measurements of the initial cathode current values in the heater structure of the prior art shown in FIG. 1. Assuming for "R" that the value of Ik is 31.3 .mu.A ( the initial current, IK.sub.1, max.) and the value of Is the reference current, IK.sub.0) is 20 .mu.A, the overshoot rate is expressed as the following: ##EQU1## And, for "B": assuming that Ik is 26.5 .mu.A (the initial current, IK.sub.1, min.) and Is is 20 .mu.A, the overshoot rate is expressed as: ##EQU2## The reason for taking a serious view of the overshoot rate is that a higher overshoot rate results in an increased initial cathode current variation, thereby leading to change in the brightness on the screen of the cathode ray tube.